1. Technical Field
The present invention relates to an oscillator circuit for generating a frequency signal and, more particularly, to a resistor-capacitor (RC) oscillator circuit using a resistor and a capacitor for generating a frequency signal.
2. Description of the Related Art
In general, oscillators display some degree of frequency variation with temperature. As with variations in temperature, oscillators may exhibit sensitivity to operating voltage. A typical RC oscillator circuit includes a plurality of inverters serially connected, a capacitor, and a resistor. The typical RC oscillator circuit generates a frequency signal having a specific frequency using an RC time constant of the capacitor and the resistor.
FIG. 1 is a circuit diagram of a conventional resistor-capacitor (RC) oscillator circuit 100. Referring to FIG. 1, the RC oscillator circuit 100 includes inverters 107, 110, 120, 130, 170, and 180, a capacitor 140 connected between nodes N_C1 and N_C2, a transistor 105 responding to an input signal IN, and a NAND device 160.
For the purposes of this description, it is assumed that a threshold voltage (switching point) of a first inverter 110 is VDD/2, where VDD is a supply voltage, and that the propagation delay time of each inverter in the RC oscillator circuit 100 is much smaller than the RC decay time constant. In the conventional RC oscillator circuit 100 shown in FIG. 1, if an input signal IN transits to high, the transistor 105 is turned off and the RC oscillator circuit 100 is enabled.
If the voltage at the node N_Cl rises from 0 V to VDD/2, the voltage at the node N_C2 rises to a supply voltage VDD after a predetermined time delay (propagation delay) by the first and second inverters 110 and 120. For example, the voltage at the node N_C1 may rise from VDD/2 to 3*VDD/2 by charge maintenance of the capacitor 140.
Since the third inverter 130 inverts and outputs the voltage of the node N_C2, a voltage of 0 V is applied to the node N_C1 by the NAND device 160 and the inverter 170. Thus, the voltage at the node N_C1 falls from 3*VDD/2 to VDD/2, which is the threshold voltage of the first inverter 110, according to the slope of the RC time constant of the resistor 150 and the capacitor 140. At that time, a frequency signal FOUT, which is output through the output node NOUT, is maintained at 0 V during a half period of the frequency signal FOUT.
However, the frequency of a frequency signal output from the RC oscillator circuit 100 of FIG. 1 may decrease at a low temperature and low supply voltage and increase at a high temperature and high supply voltage. Also, the frequency of the frequency signal output from the RC oscillator circuit 100 may increase or decrease when the current driving ability increases or decreases, for example, due to process variance.
As described above, the RC oscillator circuit 100 of FIG. 1 may exhibit sensitivity to process, voltage, and temperature variances. For example, the RC oscillator circuit 100 may exhibit frequency variation of 10% to 20% depending on process type, 20% to 30% depending on temperature (e.g., −40° to 80° C.), and 20% to 30% depending on voltage (e.g., 1.6 V to 2.0 V).